This invention is directed generally to the field of radio receivers, and more particularly to a novel noise blanking system for eliminating impulse-type noise from the audio output signal of a radio receiver, to a novel signal strength meter response linearizing system, and to a novel tuning system for a radio receiver.
The problem of adequate noise suppression with respect to a number of potential noise or spurious signal sources has been often encountered in the radio receiver arts. In a radio receiver of the type capable of receiving radio signals over multiple bands or over a relatively broad bandwidth, maintaining reception capability while rejecting noise and spurious signals invariably presents problems in receiver design. While the prior art has addressed the problem of suppression of noise or spurious signals from a variety of sources, there is room for improvement in addressing the problem of eliminating impulse-type noise from the audio output signal of a radio receiver.
Such impulse noise signals are especially bothersome in short wave (SW) receivers, which may receive signals in single side band (SSB) transmissions comprising either upper side band (USB) or lower side band (LSB) as well as continuous wave (CW) transmissions of the type generally utilized to transmit Morse code. Such impulse noise (also referred to as pulsating or pulse-type noises) may come from various sources. For example, motors, fluorescent lamps, and the like, may produce pulsating signals or noise pulses which may be picked up by a radio receiver and reproduced in the audio output signal.
Another source of such pulse-type noise particularly in short wave type receivers is the pulse noises associated with "over the horizon radar" ("the woodpecker"). Since such impulse or pulse-type noises are of relatively short duration, they are difficult to detect and eliminate from the audio output of a radio receiver. However, since such noises are often repetitive and randomly occurring, they are irritating and often obscure portions of the desired signal transmission, when they are reproduced in the audio signal output of the receiver.
Radio receivers often include a signal strength meter for producing an indication of the relative strength of the received signal. Such a signal strength indication is useful in aiding manual tuning of the received signal to attain the maximum signal strength and the optimum tuning closest to the center frequency of the received signal. However, the range of signal strength of received signals generally varies over a relatively wide range. Hence, to permit stable receiver operation and provide a reliable audio signal output well above the noise level, most receivers incorporate an automatic gain control circuit. Such an automatic gain control circuit generally controls the gain at one or more amplifiers which process the received signal. That is, the gain is increased for relatively weak received signals and decreased for relatively strong received signals. This process tends to compress the range of signal strength of the resulting demodulated or discriminated audio signals. Signal strength meters in most receivers reflect the effects of the automatic gain control circuit and hence give only an abbreviated or compressed indication of the range of signal strength of the received radio signals.
In this regard, such signal strength meters may be coupled to receive a gain control signal of the automatic gain control circuit, which bears an inverse relation to the received signal strength. However, the automatic gain control circuit signal generally has a greatly abbreviated linear range compared with the range of received signal strength. Hence, use of this signal at the meter results in a greatly abbreviated or compressed indication of received signal strength.
Heretofore, radio receiver tuning systems have generally utilized either conventional heterodyne or phase-locked loop circuits and techniques. Such techniques generally provide continuous tuning of frequencies over a given range. However, in many applications such continuous frequency tuning is not necessary. We have found that an incremental frequency tuning system can aid in eliminating spurious signals and in eliminating disturbances such as beat frequencies or "birdies".